EDITED... (I first thought you were talking about problems burning, not encoding... my fault)
I think it'd be in your best interest to research that one a little more before switching to Toast (maybe post the error message here, call Apple, etc.). As for the differences in the apps, iDVD is a lot easier to use and ends up creating a lot better output when it comes to the customized menus. Both will produce discs that can be played in DVD players, but iDVD's menu creation system is really it's strong point. I think it's worth it to stay with iDVD and get it working.
Also - there was a known issue with iDVD's encoding on the G5s when they first came out, and the fix for that was to do a full uninstall and reinstall - maybe it'll work for you too? When you went to iDVD 4, did you delete 3.0 first, or just upgrade? The fix for the G5s was to remove and replace iDVD by a) dragging iDVD out of Applications and into the trash, and b) searching for "iDVD" and deleting the config files on the system (just deleting the file com.apple.idvd.plist should be enough), and c) reinstalling iDVD from iLife '04 again on a clean system.